To determine the distribution of GnRH mRNA-expressing neurons in pigtailed macaque brain and to examine whether subpopulations of GnRH neurons express galanin mRNA as they do in rats, we used in situ hybridization and computerized imaging. GnRH mRNA was expressed in the medial forebrain and GnRH mRNA-expressing neurons were found in lateral regions of the forebrain. These results suggest that galanin stimulates luteinizing hormone secretion in primates. In addition, they show that GnRH mRNA is widely expressed in the forebrain of pigtailed macaques and that, in some populations of neurons, galanin may act as a cotransmitter with GnRH. Using the same techniques, we mapped the distribution of leptin receptor mRNA. Leptin acts as a metabolic activator of the neuroendocrine reproductive axis in several rodent species, but whether it plays a similar role in primates is unknown. We observed leptin receptor mRNA in the anterior pituitary and several areas of the brain, sug gest ing that leptin is indeed a metabolic signal to the reproductive axis in primates. We have also begun to investigate whether the use of exogenous estrogen or of selective estrogen receptor modulators affects neuropeptide gene expression. We are now testing whether estrogen receptors are coexpressed on neurons in the hypothalamus that may mediate changes in GnRH pulsatility. FUNDING NIH grants RR00166 and HD12629. Finn, P.D., Cunningham, M.J., Pau, K.Y., Spies, H.G., Clifton, D.K., and Steiner, R.A. The stimulatory effect of leptin on the neuroendocrine reproductive axis of the monkey. Endocrinology 139:4652-4662, 1998.